Data processing apparatus and method for instruction pre-decoding

ABSTRACT

The present invention provides a data processing apparatus comprising processing circuitry for executing a sequence of instructions and pre-decoding circuitry for receiving the instructions fetched from memory. The pre-decoding circuitry performs a pre-decoding operation to generate corresponding pre-decoded instructions and stores them in a cache for access by the processing circuitry. For each instruction fetched from the memory, the pre-decoding circuitry detects whether the instruction is an abnormal instruction and upon such detection provides in association with a corresponding pre-decoded instruction an identifier identifying that instruction as abnormal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to pre-decoding instructions and inparticular to identifying particular instructions when pre-decodinginstructions.

2. Description of the Prior Art

In a typical data processing apparatus, significant power is consumed indecoding instructions prior to execution within the execution pipelinesof the processing circuitry. This issue can become particularlyproblematic in processing circuitry that supports multiple instructionsets, since often multiple separate decoders will need to be providedfor decoding instructions from the various instruction sets. By way ofexample, in some implementations approximately 15% of the processorpower may be consumed by the instruction decoders.

It is typically the case that one or more caches are provided within thedata processing apparatus for caching the instructions and data requiredby the processing circuitry. At any particular level in a cachehierarchy, separate instruction and data caches may be provided (oftenreferred to as a Harvard architecture), or alternatively a unified cachemay be provided for storing the instructions and data (often referred toas a Von Neumann architecture). When instructions are fetched frommemory for storing in a cache, some known systems have employedpre-decoding mechanisms for performance orientated reasons. Inaccordance with such mechanisms, instructions are pre-decoded prior tostoring in the cache, and in such cases the cache often then storesinstructions in a wider format than the instructions stored in mainmemory, to accommodate the additional information produced by thepre-decoding process. To assist in improving performance when theinstructions are later decoded and executed, the extra informationprovided in the pre-decoded instructions as stored in the cache has beenused to identify branch instructions, identify classes of instructions(e.g. load/store instructions, coprocessor instructions, etc) to laterassist multi-issue circuitry in dispatching particular instructions toparticular execution pipelines, and to identify instruction boundariesin variable length instruction sets.

For example, the article “Performance Evaluation Of A DecodedInstruction Cache For Variable Instruction Length Computers”, IEEETransactions on Computers, Volume 43, number 10, pages 1140 to 1150,October 1994, by G Intrater et al., discusses the storing of pre-decodedinstructions in a cache. The article “The S-1 Project: DevelopingHigh-Performance Digital Computers” by L. Curtis Widdoes, Jr., LawrenceLivermore National Laboratory, 11 Dec. 1979, describes the S1 Mark IIAcomputer, where a decoded instruction cache expanded the 36-bitinstruction word to a 56-bit instruction cache format to reduceinstruction decoding time (see also the paper “Livermore S-1Supercomputer—A Short History” appearing on the websitehttp://www.cs.clemson.edu/˜mark/s1.html). Further, the idea of usingpre-decoding mechanisms to pre-identify branches and pre-identifyinstruction boundaries is discussed in the AMD K5 Processor Data sheet,Publication no. 18522E-0, September 1996, Section 4.5, Innovative x86Instruction Predecoding, page 6, which discusses adding 4 bits perinstruction byte to identify start, end, opcode position, and number ofRops (RISC operations) the individual x86 instruction requires for latertranslation.

Whilst the above-mentioned pre-decoding mechanisms can improve theperformance of the processing circuitry, they do not typicallysignificantly alleviate the earlier mentioned power cost associated withthe later decoder circuits used to decode the instructions once they areoutput from the instruction cache.

Thus, it is desirable to provide an improved pre-decoding mechanism,which can reduce the power and area cost associated with the laterdecoder circuits used to decode the instructions.

SUMMARY OF THE INVENTION

Viewed from a first aspect, the present invention provides a dataprocessing apparatus comprising: processing circuitry for executing asequence of instructions; pre-decoding circuitry for receiving theinstructions fetched from memory and performing a pre-decoding operationto generate corresponding pre-decoded instructions; a cache for storingthe pre-decoded instructions for access by said processing circuitry;wherein the pre-decode operation, for each instruction fetched from thememory, detects whether the instruction is an abnormal instruction andupon such detection provides in association with a correspondingpre-decoded instruction an identifier identifying that instruction asabnormal.

Pre-decoding instructions before storing them in an instruction cache isa technique that has previously been implemented to enhance theperformance of a data processing apparatus. However the performancebenefit of pre-decoding instructions in this manner generally comes atthe cost of greater power consumption when later fetching thoseinstructions from the instruction cache for execution, due to thegreater length of the pre-decoded instruction resulting from thepre-decoding operation. However, since fetching cache lines from mainmemory is usually a rare event compared with fetching from theinstruction cache, the inventor of the present invention realised that apre-decoder would normally operate far less on a given instruction thanthe later decoder and thus that the potential existed for a power savingin the data processing apparatus to be achieved. Yet the inventor of thepresent invention also realised that it would only be possible to reduceoverall power consumption if one could identify a feature of aninstruction for which the power consumed by accessing a widerinstruction cache is outweighed by the power saving made by not havingto decode that feature after the instruction cache.

As stated above instruction pre-decoding is generally implemented as aperformance-enhancing technique and the added power consumption isviewed as a price worth paying for the performance benefit. However, theinventor of the present invention found that the decoding circuitryrequired for the identification of abnormal instructions in the decodestage after the instruction cache could contribute significantly to thetotal gate count of the decoder. This is even more significant in asecure data processing apparatus since abnormal instructions must bedecoded in a consistent, deterministic way so that no matter whatcondition the processor is in and no matter what instructions it hasrecently executed it will always behave in the same way with a specificabnormal instruction. Because abnormal instructions must be decoded in aconsistent, deterministic way, more gates must be put into the decoderto deal with corner cases in the instruction set architecture. Byshifting this identification to the pre-decode stage and providing anabnormal instruction identifier in association with the pre-decodedinstruction, the inventor found that the added power consumption of thisarrangement was outweighed by the power saving of not having to identifyabnormal instructions at the decode stage.

An abnormal instruction may take a variety of forms, for example in oneembodiment said abnormal instruction is an undefined instruction. Theinstruction space available for defining instructions recognised by thedata processing apparatus will typically not all be used, and hence ifan encoding in this unused space is presented, that encoding will beconsidered to be an undefined instruction. The abnormal instruction mayalso be an unpredictable instruction. An unpredictable instruction is aninstruction that is a defined instruction, yet the manner in which it isused will not result in a well-defined outcome in the data processingapparatus (for example, a load or store instruction which attempts toperform a base register writeback with the base set to the programcounter register). Whilst this might not lead to any negativeconsequences, the unpredictability of the outcome means that suchinstructions must be identified and carefully handled, in order toguarantee the integrity of the data processing apparatus.

It will be appreciated that there are a variety of ways that thepre-decoding circuitry could provide the identifier, but in oneembodiment of the present invention, said identifier comprises side bandbits in said corresponding pre-decoded instruction. These side band bitscould, for example, be appended to said corresponding pre-decodedinstruction, be inserted into said corresponding pre-decoded instructionor replace redundant bits in said corresponding pre-decoded instruction.Whatever their form, the side band bits, added to the instruction by thepre-decoding circuitry, represent a simple manner of providing anindication of whether a pre-decoded instruction is an abnormalinstruction or not.

The identifier may be configured to simply indicate that the instructionis abnormal, but in one embodiment said identifier identifies the typeof the abnormal instruction, and hence will identify whether theabnormal instruction is an undefined instruction or is an unpredictableinstruction.

Whilst the identifier may give a direct indication of the type ofabnormal instruction, in one embodiment, said identifier identifies saidabnormal instruction as undefined irrespective of the type of theabnormal instruction. For example, although recognised as anunpredictable instruction, the pre-decoding circuitry may mark theinstruction as an undefined instruction, since this provides a simplemechanism for triggering a well-defined response to an abnormalinstruction in the processing circuitry.

As an alternative to, or in addition to, providing side band bits toidentify an abnormal instruction, in one embodiment the pre-decodeoperation converts said abnormal instruction into a predeterminedundefined instruction. Thus, whatever the abnormal instruction, becauseit is converted into a predetermined instruction which is undefined, anadvantageously simple mechanism for handling just one undefinedinstruction may be provided. If side band bits are not used, then thepredefined undefined instruction format itself forms the identifieridentifying the instruction as abnormal.

In one embodiment of the present invention, upon encountering saidcorresponding pre-decoded instruction said processing circuitry isconfigured to execute said abnormal instruction in a predeterminedmanner. This predetermined manner helps to ensure that the dataprocessing apparatus responds to abnormal instructions in an predictablemanner.

There are clearly many predetermined manners in which the processingcircuitry could execute the abnormal instruction, but in one embodimentsaid predetermined manner comprises executing said correspondingpre-decoded instruction such that a state of the data processingapparatus is not altered. This “no-operation” style of response ensuresthat whatever the abnormal instruction, it has no effect on the internalstate of the data processing apparatus, for example the registercontents used by the data processing apparatus. In another embodiment,said predetermined manner comprises triggering an exception. In thisway, a dedicated exception routine can be provided for responding to anabnormal instruction, which can be flexibly defined, depending on therequirements of the particular data processing apparatus.

Viewed from a second aspect, the present invention provides a method ofprocessing data comprising the steps of: fetching instructions frommemory; performing a pre-decoding operation on said fetched instructionsto generate corresponding pre-decoded instructions, wherein saidpre-decoding operation comprises, for each instruction fetched from thememory, detecting whether the instruction is an abnormal instruction andupon such detection providing in association with a correspondingpre-decoded instruction an identifier identifying that instruction asabnormal; and storing the pre-decoded instructions in a cache for accessby said processing circuitry.

Viewed from a third aspect, the present invention provides a dataprocessing apparatus comprising: processing means for executing asequence of instructions; pre-decoding means for receiving theinstructions fetched from memory means and performing a pre-decodingoperation to generate corresponding pre-decoded instructions; cachemeans for storing the pre-decoded instructions for access by saidprocessing means; wherein the pre-decode operation, for each instructionfetched from the memory means, detects whether the instruction is anabnormal instruction and upon such detection provides in associationwith a corresponding pre-decoded instruction an identifier identifyingthat instruction as abnormal.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described further, by way of example only,with reference to embodiments as illustrated in the accompanyingdrawings, in which:

FIG. 1 schematically illustrates a data processing apparatus accordingto one embodiment;

FIG. 2 is a flow diagram illustrating a series of steps carried out by adata processing apparatus according to one embodiment;

FIG. 3 is a flow diagram illustrating in more detail a series of stepscarried out by pre-decoding circuitry when providing an abnormalindicator;

FIG. 4 is a flow diagram illustrating a series of steps carried out byprocessing circuitry when fetching and executing instructions;

FIG. 5 illustrates one particular example embodiment of the dataprocessing apparatus;

FIG. 6 illustrates two instruction formats prior to pre-decoding takingplace;

FIG. 7 illustrates how data representing program instructions can bestored in memory space;

FIGS. 8A to 8D illustrate several possible situations in whichpre-decoding errors could arise;

FIG. 9 illustrates the format of two example instructions afterpre-decoding in accordance with one embodiment;

FIG. 10 is a table listing situations where pre-decoding errors may ormay not occur;

FIG. 11 is a flow diagram illustrating the operation of the dataprocessing system in some embodiments;

FIG. 12 illustrates the use of an incomplete field within a pre-decodedThumb 32 instruction in accordance with one embodiment;

FIG. 13 illustrates two possible routes through the data processingapparatus for a Thumb 32 instruction that crosses a cache line boundary,in accordance with one embodiment;

FIG. 14 is a flow diagram illustrating a pre-decoding operation sequencein accordance with one embodiment;

FIG. 15 illustrates the format of two example instructions afterpre-decoding in accordance with one embodiment;

FIG. 16 is a flow diagram illustrating in more detail the stepsperformed to implement step 202 of FIG. 11 in accordance with oneembodiment;

FIG. 17 illustrates components provided within the instruction cache inaccordance with one embodiment;

FIG. 18 illustrates how each entry of the tag RAM of the cache isextended to store speculative processor state in accordance with oneembodiment;

FIG. 19 schematically illustrates the construction of a two-way setassociative instruction cache as used in one embodiment;

FIG. 20 is a flow diagram illustrating the pre-decoding operationaccording to one embodiment;

FIG. 21 illustrates a cache lookup procedure in accordance with oneembodiment;

FIG. 22 illustrates one embodiment where the pre-decoder circuitry isassociated with a combined level two cache within the data processingapparatus;

FIG. 23 illustrates the use of an additional instruction/data identifiervalue in association with each cache line in the combined level twocache of FIG. 22;

FIG. 24 schematically illustrates a portion of a processing pipelinehaving decoding circuitry including a shared portion used to decodepre-decoded instructions corresponding to instructions in differentinstruction sets;

FIG. 25 illustrates how different portions of an ARM instruction may bepre-decoded into a pre-decoded instruction having a shared format withThumb32 instructions;

FIG. 26 illustrates how a Thumb32 and an ARM instruction canrespectively be pre-decoded into pre-decoded instructions having ashared format to represent shared functionality;

FIG. 27 illustrates how an ARM instruction may be pre-decoded into apre-decoded instruction corresponding to an unused portion within theThumb32 instruction bitspace;

FIG. 28 illustrates how two adjacent Thumb16 instructions may beconcatenated and pre-decoded into a pre-decoded instructioncorresponding to a single Thumb32 instruction;

FIG. 29 is a flow diagram schematically illustrating the re-mapping ofARM instructions to Thumb32 instructions;

FIG. 30 illustrates how two Thumb16 instructions can be re-mapped duringpre-decoding to a pre-decoding instruction corresponding to a singleThumb32 instruction; and

FIG. 31 is a flow diagram schematically illustrating how branchinstructions can be identified during pre-decoding and used to triggerearly termination of pre-decoding.

DESCRIPTION OF EMBODIMENTS

FIG. 1 schematically illustrates a data processing system 1 according toone embodiment. The data processing system 1 includes a processor systeminterface 5 which contains instruction fetching circuitry for fetchingdata representing program instructions from memory (not shown). Theprocessor system interface 5 passes instructions fetched from memory topre-decoding circuitry 10. The pre-decoding circuitry 10 performspre-decoding operations on the instructions to generate pre-decodedinstructions, which are passed to a line-fill buffer 15 and stored in aninstruction cache 20. Instructions stored in the instruction cache 20are executed by processing circuitry 25. The processing circuitry 25includes decoding circuitry 30 for generating control signals from thepre-decoded instructions which control the processing circuitry so as toperform processing operations. The error detection circuitry 35 isemployed to detect certain errors occurring in the pre-decodedinstructions and to cause the relevant instructions, or the cache linescontaining those instructions, to be re-passed through the pre-decodingcircuitry 10 (either directly or via initiation of a linefill operation)before being supplied to the processing circuitry 25. In the event sucherrors are detected, an error signal is sent from the error detectioncircuitry 35 to the processing circuitry 25 to cause the decodingcircuitry 30 to cease any decoding that may have begun in respect ofsuch pre-decoded instructions for which an error has been detected, theprocessing circuitry 25 instead awaiting the pre-decoded instructionobtained by the re-pass through the pre-decoding circuitry 10.

Pre-decoding instructions before storing them in an instruction cache isa technique that has previously been implemented to enhance theperformance of a data processing apparatus. However the performancebenefit of pre-decoding instructions in this manner generally comes atthe cost of greater power consumption when later fetching thoseinstructions from the instruction cache for execution, due to thegreater length of the pre-decoded instruction resulting from thepre-decoding operation. However, since fetching cache lines from mainmemory is usually a rare event compared with fetching from theinstruction cache, it has been realised that a pre-decoder wouldnormally operate far less on a given instruction than the later decoderand thus the potential existed for a power saving in the data processingapparatus to be achieved. Yet it was also realised that it would only bepossible to reduce overall power consumption if one could identify afeature of an instruction for which the power consumed by accessing awider instruction cache is outweighed by the power saving made by nothaving to decode that feature after the instruction cache.

As stated above instruction pre-decoding is generally implemented as aperformance-enhancing technique and the added power consumption isviewed as a price worth paying for the performance benefit. However, ithas been found that the decoding circuitry required for theidentification of abnormal instructions in the decode stage after theinstruction cache could contribute significantly to the total gate countof the decoder. This is even more significant in a secure dataprocessing apparatus since abnormal instructions must be decoded in aconsistent, deterministic way so that no matter what condition theprocessor is in and no matter what instructions it has recently executedit will always behave in the same way with a specific abnormalinstruction. Because abnormal instructions must be decoded in aconsistent, deterministic way, more gates must be put into the decoderto deal with corner cases in the instruction set architecture. Byshifting this identification to the pre-decode stage and providing anabnormal instruction identifier in association with the pre-decodedinstruction, it has been found that the added power consumption of thisarrangement was outweighed by the power saving of not having to identifyabnormal instructions at the decode stage.

FIG. 2 schematically illustrates a series of steps carried out by dataprocessing apparatus 1 when performing a pre-decode operation inaccordance with one embodiment. The flow begins at step 300 withinpre-decoding circuitry 10, an instruction having been retrieved frommemory (via the processor system interface 5). At step 302 it isestablished if the instruction is abnormal or not. In one embodiment anabnormal instruction is defined to be one that is either undefined orunpredictable. An undefined instruction is one that is within theinstruction space, yet not part of the instruction set. There canclearly be a great number of such undefined instructions, since in a32-bit (for example) instruction space there are 2³² (over 4 billion)possible encodings. On the other hand, an unpredictable instruction isone which is a defined instruction, yet is being used in a manner suchthat it will not result in a well-defined outcome in the data processingapparatus, for example a load or store instruction which attempts toperform a base register writeback with the base set to the programcounter register.

If the instruction is identified as abnormal, then at step 304pre-decoding circuitry 10 provides an identifier identifying thatinstruction as abnormal. This identifier is associated with thepre-decoded instruction that the pre-decoding circuitry 10 generates, inone embodiment side band bits in the pre-decoded instruction serving asthe identifier.

If however at step 302 the instruction is identified as not beingabnormal, then step 304 is omitted. The flow proceeds to step 306 wherepre-decoding circuitry 10 performs any other pre-decode steps that arerequired (examples of other pre-decode steps are given later in thedescription). Finally, at step 308 the pre-decoded instruction is storedin instruction cache 20 (via linefill buffer 15).

FIG. 3 schematically illustrates in more detail a series of stepscarried out in one embodiment by pre-decoding circuitry 10 in step 304of FIG. 2. Firstly, at step 320 it is checked if the pre-decodingcircuitry 10 is currently configured to convert all abnormalinstructions into a predetermined undefined instruction. If so, then theflow proceeds to step 324, where this conversion takes place. Inembodiments of the present invention in which the pre-decoding circuitryis configured to do this, this allows the mechanism for handlingabnormal instructions in the processing circuitry to be particularlysimple, since only one particular undefined instruction will ever bereceived by the processing circuitry, and its response (for example tocall a dedicated exception routine) is then well-defined and predictablefor all abnormal instructions. From step 324, the flow continues to step306 in FIG. 2. Hence in this embodiment, the use of the pre-determinedundefined instruction itself provides the “indicator” of the abnormalinstruction.

Otherwise the flow proceeds to step 326 where it is checked if thepre-decoding circuitry 10 is currently configured to label all abnormalinstructions as undefined. This configuration of pre-decoding circuitry10 also simplifies the downstream mechanisms for handling abnormalinstructions, since all abnormal instructions are then handled as ifthey are undefined instructions and the processing circuitry's responseis then well-defined and predictable for all abnormal instructions (forexample, such instructions could trigger an exception, or alternativelycould be NOP (no-operation) handled, passing through the processingcircuitry without having any effect on the internal state of theprocessing circuitry as a result). If all abnormal instructions shouldbe labelled as undefined, then the flow proceeds to step 330, where thecorresponding indicator is generated, in this case adding side band bitsto the pre-decoded instruction indicating that the instruction isundefined.

If the pre-decoding circuitry 10 is not currently configured to labelall abnormal instructions as undefined, then the flow proceeds to step328, where it is checked if the identified abnormal instruction is anundefined instruction. If so, the flow proceeds to step 330 and sideband bits are added to the pre-decoded instruction indicating that theinstruction is undefined. If the abnormal instruction is not undefined,then in this embodiment it must be unpredictable, and at step 332 sideband bits are added to the pre-decoded instruction indicating that theinstruction is unpredictable. All paths through FIG. 3 finally merge toproceed to step 306 in FIG. 2.

FIG. 4 illustrates a series of steps carried out by processing circuitry25 when fetching and executing instructions that may have an associatedindicator labelling the pre-decoded instruction as abnormal. At step 350a pre-decoded instruction is fetched from the instruction cache 20 intothe processing circuitry 25.

Next, at step 352 the pre-decoded instruction is examined to establishif it has been labelled as abnormal, by means of an associatedindicator. If the pre-decoded instruction is not so labelled asabnormal, then at step 354 normal execution (preceded by furtherdecoding by decoding circuitry 30 as necessary) of the instruction iscarried out.

On the other hand, if the instruction is abnormal then the processingcircuitry 25 executes the instruction in a predetermined manner (step356). In this embodiment the processing circuitry is configured to dothis in one of two ways. Firstly, at step 358, if the processingcircuitry is configured to NOP the instruction, then at step 360 theinstruction is executed as NOP, passing through the processing circuitrywithout any state of the processing circuitry changing as a result.Alternatively, if the processing circuitry is configured to trigger anexception in response to an abnormal instruction, then at step 362 asuitable exception is called.

Hence, according to the above described embodiment, a data processingapparatus is provided in which power savings may be made by identifyingabnormal instructions at a pre-decode stage. By generating an identifiershowing that an instruction is abnormal, when the pre-decodedinstruction is later retrieved from an instruction cache, the processingcircuitry may more swiftly handle the abnormal instruction. Furthermore,the early identification of abnormal instructions enables the provisionof a data processing apparatus which responds in a predictable andwell-defined manner to instructions which could otherwise jeopardise theoperation and security of the device.

The data processing system 1 processes instructions from one or moreinstruction sets. For example, some processors produced by ARM Limitedof Cambridge may execute instructions from the ARM instruction set, aswell as from the mixed-length Thumb 2 instruction set. Thumb 2instructions may be 16 bits or 32 bits long. Thus, the decodingcircuitry 30 can include decoders for each set of instructions. FIG. 5shows an example of the data processing apparatus 1 in which thedecoding circuitry 30 includes an ARM decoder 40, a Thumb-32 decoder 45and a Thumb-16 decoder 50. The decoders 40, 45, 50 are used to decodepre-decoded instructions stored in the instruction cache 20. Thedecoders 40, 45, 50 generate control signals from the pre-decodedinstructions which control the processing circuitry so as to performprocessing operations. A multiplexer 65 selects which of the decoders40, 45, 50 is used depending on the processor state.

FIG. 6 shows a Thumb-16 (T16) instruction 100 and a Thumb-32 (T32)instruction 110 before they are pre-decoded. A T16 instruction 100consists of a 16-bit opcode (including all fields). A T32 instruction110 consists of two portions. The first portion T32a consists of16-bits. Bits [15:13] in the first portion T32a of a T32 instruction 110are set to 0b111. Bits [12:0] of the first portion T32a store the first13 bits of the T32 opcode. The second portion T32b of a T32 instruction110 contains 16 bits which comprise the second half of the T32 opcode.For T16 and T32 instructions, the two portions of the T32 are the samelength as each other, and the same length as a T16. However, this is notnecessarily the case for all mixed-length instruction sets.

FIG. 7 shows how data representing program instructions can be stored inthe memory space 150. Data is arranged in blocks 170, each blockrepresenting a portion of a program instruction. For example, each blockmay represent a T16 instruction, the first portion T32a of a T32instruction or the second portion T32b of a T32 instruction. Blocks arearranged in cache lines 160. For simplicity, in FIG. 7 a cache line 160is depicted as containing four blocks, but it will be appreciated that acache line 160 can store other (typically large) numbers of blocks 170.A T32 instruction may span the boundary between two cache lines 160, sothat the last block of one cache line represents T32a and the firstblock of the following cache line represents T32b.

When a program is executed, the processing circuitry executesinstructions stored in the instruction cache 20 if possible. If aninstruction is not present in the instruction cache 20, then a cachemiss occurs and blocks of data representing the instruction are fetchedfrom memory. The fetching circuitry within the processor systeminterface 5 fetches a cache line 160 including the block or blocksrepresenting the desired instruction from memory. The fetched blocks arepre-decoded by the pre-decoding circuitry 10 before being placed in theinstruction cache 20 ready for further decoding. However, when blocksare fetched, the pre-decoding circuitry 10 cannot determine withcertainty which portion of an instruction a block represents. Thus, thepre-decoding circuitry 10 makes a speculative identification of whichportion of an instruction a fetched block represents.

The instruction blocks may have some bits which give an indication ofwhich portion a block is likely to represent. For example, with themixed-length Thumb2 instructions a block representing the first portionT32a of a T32 instruction will have bits [15:13] set to 0b111. Apre-decoding operation is performed on a block, depending on theidentification made by the pre-decoding circuitry 10. For example, anabnormal instruction identifier may be added to the block indicatingwhether or not the block is an abnormal instruction. However the samebit pattern could potentially also arise for the second portion T32b ofa T32 instruction, depending on the value of the opcode for thatinstruction. Thus, the speculative identification may be incorrect. Ifan incorrect identification of a block is made, then the pre-decodingcircuitry 10 may perform the wrong pre-decoding operation on that block,resulting in a pre-decoding error.

FIGS. 8A to 8D illustrate several possible situations in which errorscould arise. FIG. 8A shows an error which can occur when the processorbranches into the middle of a cache line. If a cache miss occurs, theprocessor system interface fetches the cache line from memory. Thepre-decoding circuitry 10 starts pre-decoding from the point at whichthe program branches to so that the processor may unstall as soon aspossible. The pre-decoding circuitry 10 then continues pre-decodingblocks up to the end of the cache line. The pre-decoding circuitry 10then returns to the beginning of the cache line, makes a speculativeidentification of which portion the first block in the cache linerepresents, and pre-decodes the remaining blocks in the cache line. Thespeculative identification of the first block in the cache line may beincorrect, causing a pre-decoding error. For example, in FIG. 8A, thefirst block of in the cache line, which actually represents a T32bportion, could be mistakenly identified as a T32a portion or a T16portion, depending on the value of bits [15:13] of the block.

Another type of error can occur when a branch misprediction occurs,causing the program flow to branch into the middle of a T32 instructionby mistake, as illustrated in FIG. 8B. In this case the pre-decodingcircuitry 10 may incorrectly identify the following T32b portion as aT16 instruction or as a T32a. Subsequent blocks may also bemisidentified. For example, if the block at the branch point, whichactually represents a T32b portion, is mistakenly identified as a T32a,then the following block will be identified as a T32b. This could causefurther errors.

If a T32 instruction spans a cache line boundary, as illustrated in FIG.8C, an error may occur when the pre-decoding circuitry 10 decodes thesecond cache line. The T32b portion may be identified as a T16instruction or as a T32a.

FIG. 8D illustrates an error which may occur if the cache line containsdata which does not represent a program instruction (a literal). In thiscase the pre-decoding circuitry 10 may interpret the literal as aninstruction portion. This may cause a knock on effect in theidentification of subsequent blocks. For example, if the pre-decodingcircuitry 10 identifies the literal as a T32a portion, it willpre-decode the following block as a T32b portion of the T32 instruction.

Thus, there are several situations in which pre-decoding errors mayarise. Therefore, in embodiments the data processing system 1 isprovided with error detection circuitry 35 which can detect if thespeculative identification made by the pre-decoding circuitry 10 was orwas not correct and if not, signal a pre-decoding error and correct theerror instructions in the cache 20. Pre-decoding errors may be correctedby sending at least part of pre-decoded instructions for which errorshave been detected back to the pre-decoding circuitry 10 to bepre-decoded again. In some situations the pre-decoding circuitry 10 maybe able to regenerate the original instruction blocks from thepre-decoded blocks. An alternative method of correcting errors is toinvalidate a cache line in the cache 20 which contains an error andcause the cache line to be re-fetched from memory and pre-decoded again.

In order for the error detecting circuitry to be able to detect as whichportion the pre-decoding circuitry 10 speculatively identified a block,the pre-decoding circuitry marks each block with an instruction portionidentifier that indicates as which portion the block has beenidentified. The instruction portion identifier is added to a block asone or more additional bits.

In one embodiment, the pre-decoding operation is performed in order toadd an abnormal instruction identifier to a T32 instruction indicatingwhether or not the instruction is abnormal or not. This abnormalinstruction identifier is added to the first portion T32a of the T32instruction in the form of sideband information. In this embodiment, nosideband information is added to the second portion T32b of the T32instruction or the T16 instruction.

Thus, the T16 opcode and the second half of the T32 opcode areunmodified by the pre-decoding circuitry 10. This means that it does notmatter if the pre-decoding circuitry 10 mistakenly identifies a blockrepresenting a T16 instruction as the second portion T32b of a T32instruction, because the opcode will be unmodified and so the blockstored in the instruction cache 20 will be the same regardless of theidentification made. Similarly, it does not matter if the pre-decodingcircuitry 10 identifies a block which actually represents a T32b as aT16. Again, no pre-decoding error will arise from an incorrectidentification. This means that the number of pre-decoding errors whichoccur can be reduced. If fewer pre-decoding errors occur, then the errordetection circuitry 35 needs to correct fewer errors and thepre-decoding circuitry 10 does not need to pre-decode as manyinstructions more than once. Thus, the power consumed by the dataprocessing system 1 will be reduced.

As it does not matter whether a block is identified as a T16 or a T32b,the pre-decoding circuitry 10 adds the same instruction portionidentifier to a block for each case. Thus, the instruction portionidentifier needs only to identify whether or not a block represents thefirst portion T32a of a T32 instruction. This means only one bit isrequired for the instruction portion identifier, and so the instructiondoes not need to be unduly extended. This means less power is consumedin storing pre-decoded instructions in the cache 20. In one embodiment,the instruction portion identifier is set to “1” when a block isidentified as representing a T32a, and set to “0” when a blockrepresents either a T32b or a T16 instruction.

FIG. 9 shows the format of the T16 and T32 instructions afterpre-decoding in one example embodiment. If a block is identified asrepresenting the first portion T32a of a T32 instruction, thepre-decoding circuitry 10 adds an instruction portion identifier (IDbit) at bit [16] and sets the instruction portion identifier to “1”.Sideband information is inserted indicating whether or not theinstruction is an abnormal instruction. Since the instruction portionidentifier at bit [16] now indicates that the block represents a T32aportion of a T32 instruction, bits [15:13] are redundant for identifyingthe block. Thus, sideband information can be inserted into these bitswithout loss of information. One way of doing this is do insert sidebandbits in bits [15:14] of the T32a portion, one bit indicating whether ornot the instruction is undefined and one bit indicating whether or notthe instruction is unpredictable. Thus, sideband information may beadded to the block without extending the length of the block, reducingthe power consumed in storing the instructions in the cache 20.

As sideband information for a T32 instruction is only added to the firstportion T32a of the instruction, this allows the pre-decoding operationperformed on the second portion T32b to be the same as for a T16instruction, so that the number of errors which occur can be reduced. Ifa block is identified as representing a T16 instruction, thepre-decoding circuitry 10 adds an instruction portion identifier (IDbit) at bit [16] and sets the instruction portion identifier to 0. TheT16 opcode in bits [15:0] is not modified. If a block is identified asrepresenting the second portion of a T32 instruction, then the samepre-decoding operation is performed on the block as for a T16instruction. Bit [33] of the T32 instruction (corresponding to bit [16]of the block representing the second portion T32b of the T32instruction) is set to 0. The remainder of the second half of the T32opcode is not modified.

Thus, the pre-decoding circuitry does not need to detect errors when aT16 has been identified as a T32b or vice versa, and so has fewer errorsto detect. The only errors which need to be detected are when a T32bportion is incorrectly speculatively identified as a T32a portion, whentwo consecutive blocks are marked as a T32a portion (this may occur whena T32 instruction spans a cache line boundary), or when a T32a portionis mistakenly identified as a T32b portion (caused by a knock on effectfrom a mis-identification of a previous block). FIG. 10 shows a tablelisting the situations in which errors may or may not occur. The columnNative Instruction Stream shows which portion a sequence of blocks in acache line actually represent. The column Pre-decode Speculation showsas which portion the pre-decoding circuitry has speculatively identifiedthe blocks. Some pre-decoding errors may occur when a branch mispredictoccurs causing the program flow to branch into the middle of a T32instruction. Other errors may occur when the flow continues from aprevious cache line.

It will be appreciated that the present technique is not restricted toinstructions within the Thumb2 instruction set, but is applicable toinstructions from any variable-length instruction set, or toinstructions from instruction sets of differing lengths. The presenttechnique can be implemented by ensuring that for at least one portionof an instruction from a first set of instructions and at least oneportion of an instruction from a second set of instructions thepre-decoding operation performed on a block is the same regardless of aswhich of the portions the pre-decoding circuitry 10 identifies theblock. Thus, the power consumed by the system can be reduced.

FIG. 11 shows a flow chart illustrating the operation of the dataprocessing system 1 in some embodiments. Firstly, in step 200, theinstruction fetch circuitry within the processor system interface 5fetches a cache line from memory comprising a plurality of blocks ofdata representing program instructions. Next, in step 202 the blocks ofdata are passed to the pre-decoding circuitry 10, which pre-decodes theblocks by performing pre-decoding operations on the blocks and generatespre-decoded blocks. The pre-decoding operations will be described inmore detail with reference to FIG. 15. In step 204 pre-decodedinstructions made up of pre-decoded blocks are stored in the instructioncache 20. Subsequently, in step 206 the error detection circuitry 35checks for errors in the pre-decoded blocks stored in the cache 20. Instep 208 the error detection circuitry 35 identifies whether or not anypre-decoding errors have been detected.

If the error detection circuitry 35 identifies in step 208 that apre-decoding error has been detected, the error detection circuitry 35acts to correct the pre-decoding error. In one embodiment, the errordetection circuitry 35 in step 210 sends at least part of anyincorrectly pre-decoded blocks back to the pre-decoding circuitry 10.Flow then returns to step 202, in which the pre-decoding circuitry againpre-decodes the incorrectly pre-decoded blocks.

As an alternative to step 210, the error detection circuitry 35 mayinstead perform step 212. In step 212, the error detection circuitry 35invalidates the cache line in the instruction cache 20, which containsthe incorrectly pre-decoded instruction. Flow then returns to step 200,in which the cache line containing the incorrectly pre-decodedinstruction is re-fetched from memory.

If, on the other hand, the error detection circuitry 35 identifies instep 208 that no errors have been detected, flow passes to step 216. Instep 216, the decoding circuitry 30 within the processing circuitry 25decodes the pre-decoded instructions and generates control signals.Next, in step 220 the processing circuitry performs processingoperations in response to the control signals. The process then comes toan end.

Another pre-decode error that can occur, and hence should be identified,is when a Thumb-32 instruction crosses a cache line boundary, and thesecond half-word is not available to the pre-decoding circuitry at thetime the pre-decoding operation is performed. In normal operation, thepre-decoder can correctly pre-decode a Thumb-32 instruction that crossesa cache line boundary providing it has the first half-word from theprevious cache line and the second half-word from the new cache line.However, when a Thumb-32 instruction crosses a cache line boundary andthe new cache line has not been fetched at the time the pre-decodingoperation is being performed, the pre-decoding operation will beincomplete and accordingly the sideband signals will not be pre-decodedcorrectly.

In order to enable the processing circuitry to later determine, whenreading a pre-decoded instruction from the instruction cache, that thepre-decoding operation was incomplete, in one embodiment an incompletefield is added to the pre-decoded instruction to form an incompletepre-decode identifier, this field being set if the pre-decodingcircuitry does not have access to both portions of the Thumb-32instruction when performing the pre-decoding operation, and in oneparticular embodiment the pre-decoded Thumb-32 instruction can berepresented as shown in FIG. 12. In particular, when comparing FIG. 12with the earlier-discussed FIG. 9, it will be seen that each pre-decodedinstruction portion has been extended from 17 to 18 bits, so that bits 0to 17 represent the first pre-decoded instruction portion, and bits 18to 35 represent the second pre-decoded instruction portion. Bits 0 to 16of the first pre-decoded instruction portion are the same as in theearlier discussed FIG. 9, with the sideband signals (identifying thepresence of an abnormal instruction) being inserted in bit positions 14and 15, and with the instruction portion identifier being inserted atbit position 16. Further, bit position 17 is used to provide theincomplete field, this bit being set if the pre-decoding circuitry didnot have access to the second half of the Thumb-32 instruction whenperforming the pre-decoding operation, thus indicating that the sidebandsignals in bit position 14 and 15 cannot be relied upon, and may in factbe incorrect.

Considering the second half of the pre-decoded instruction, bits 17 to34 are the same as bits 16 to 33 of the earlier described embodimentshown in FIG. 9. Bit position 35 is added so as to make the secondpre-decoded instruction portion the same length as the first pre-decodedinstruction portion, and in one embodiment bit 35 is set equal to 0.However, as will be discussed in more detail later, in an alternativeembodiment bit position 35 may be used for another purpose.

In an alternative embodiment, rather than employing the incomplete fieldin bit position 17, the pre-decoding circuitry may be arranged insituations where it only has access to the first half-word of the Thumb32 instruction when performing the pre-decoding operation, to set theinstruction portion identifier in bit position 16 to identify that firstpre-decoded instruction portion as relating to a Thumb 16 instruction,i.e. by setting bit position 16 to a logic 0 value. If such apre-decoded instruction portion is later fetched from the instructioncache, the error detection circuitry 35 would observe that theinstruction portion is actually part of a Thumb 32 instruction that hasbeen incorrectly marked as a Thumb 16 instruction, and will accordinglysignal an error, preventing the potentially corrupt pre-decodedinstruction being executed by the processing circuitry. Accordingly,through this mechanism, the processing circuitry can be prevented frommaking use of an incompletely pre-decoded instruction without the needfor a separate incomplete bit as shown in FIG. 12.

As another alternative embodiment to the embodiment shown in FIG. 12,the incomplete pre-decode identifier can be stored within the cache inassociation with the tag RAM entry for a particular cache line. Whensuch an identifier is set (preferably this being provided as a singlebit field), this will flag that the pre-decoded instruction portionappearing at the end of the cache line was subjected to an incompletepre-decode operation, and hence cannot be relied upon to have beenpre-decoded correctly.

In some embodiments, it may be more space efficient to provide thissingle identifier bit in association with each tag RAM entry, ratherthan seeking to accommodate that extra piece of information in eachpre-decoded instruction.

FIG. 13 illustrates the two possible paths that a Thumb 32 instructioncrossing a cache line boundary may take through the data processingapparatus. As shown in FIG. 13, a buffer 380 is provided for storing thelast half word of a particular cache line fetched from the memory systemvia the processor system interface 5, this buffered information beingused if that last half word was the first half word of a Thumb 32instruction. If the processor is continuously fetching consecutive cachelines from memory, then by buffering this last instruction portion, thepre-decoding circuitry 10 can wait for the next cache line to be routedfrom memory to the pre-decoding circuitry 10 via the processor systeminterface 5 and the multiplexer 60, and at that point can perform thefull pre-decode operation and generate the appropriate sideband signalsusing both half words of the instruction spanning the cache lineboundary. As a result, when the pre-decoded instruction then passesthrough the linefill buffer 15 into the instruction cache 20, it cansubsequently be read from the instruction cache and passed directlythrough the decoding circuitry 30, as indicated schematically by thepath “A” in FIG. 13.

However, if the processor is not continuously fetching consecutive cachelines, the pre-decoded instruction portion corresponding to the firsthalf of a Thumb 32 instruction crossing a cache line boundary will besubjected to an incomplete pre-decoding operation, and accordingly theincomplete pre-decode identifier will need to be set, using any of theabove-mentioned three possible techniques. In such instances, when aThumb 32 instruction including such a first pre-decoded instructionportion is read from the instruction cache 20, the pre-decode errordetection circuitry 35 will detect the setting of the incompletepre-decode identifier, and in one embodiment will cause the entire Thumb32 instruction to be re-routed back via the multiplexer 60 into thepre-decoding circuitry 10, as indicated schematically by the path “B” inFIG. 13. In particular, a control signal is routed back to themultiplexer 60 from the pre-decode error detection circuitry 35 overpath 37 to cause the multiplexer to select the instruction datapresented over the loopback path 39. In embodiments of the presentinvention, this is possible due to the way the information created bythe pre-decoding operation is added into the instruction when creatingthe pre-decoded instruction. In particular, the sideband signals areadded into bit positions 15 and 14 because those bit positions becomesredundant once the instruction portion identifier is inserted in bitposition 16. As a result, it is possible for the original instruction tobe recreated readily from the pre-decoded instruction, and accordinglythe instruction can be rerouted through the pre-decoding circuitry 10without needing to re-fetch the instruction from memory.

In an alternative embodiment, if the original instruction cannot berecreated from the pre-decoded instruction, then the pre-decode errordetection circuitry 35 can be arranged to cause the relevant twoadjacent cache lines in the instruction cache to be invalidated, and alinefill operation to be performed to cause those two cache lines to berefetched sequentially from memory and passed through the pre-decodingcircuitry.

It will be appreciated that following the detection of the error andrepassing of the instruction through the pre-decoding circuitry, theinstruction will then follow path A and can be retrieved into thedecoding circuitry 30 when required by the processing circuitry.

FIG. 14 schematically illustrates the operation of the pre-decodingcircuitry in accordance with the above-described technique. Thepre-decoding operation starts at step 400, and at step 402 it isdetermined whether the instruction is a Thumb 32 instruction. If it is,it is then detected at step 404 whether that Thumb 32 instructioncrosses a cache line boundary, and if it does it is then determined atstep 406 whether the processor is fetching from sequential cache lines.

If the processor is fetching from sequential cache lines, then the firstinstruction portion of the Thumb 32 instruction appearing at the end ofthe cache line is buffered in the register 380 at step 410, whereafterat step 412 the pre-decoding circuitry 10 awaits the receipt of the nextcache line from the processor system interface 5. Once the next cacheline is available, then the pre-decoding operation is performed on theThumb 32 instruction at step 414 in the usual manner. The process willalso proceed directly to step 414 if it is detected at step 402 that theinstruction is not a Thumb 32 instruction, since in one embodiment ofthe present invention the instructions of any other instruction setsexecuted by the processor will be aligned with cache line boundaries.Similarly, if the instruction is a Thumb 32 instruction, but at step 404it is determined that the Thumb 32 instruction does not cross the cacheline boundary, then again the processor proceeds directly to step 414.

If at step 406 it is determined that the processor is not fetching fromsequential cache lines, then the process proceeds to step 408 where anincomplete pre-decoding operation is performed in respect of the firsthalf word of the Thumb 32 instruction, and the incomplete field is setto identify that the sideband signals added into the pre-decodedinstruction portion have been produced using an incomplete pre-decodingoperation.

From the above description, it will be seen that by providing such anincomplete pre-decode indication in association with one or morepre-decoded instruction portions, if a pre-decoded instruction includingsuch a pre-decoded instruction portion is later read from the cache bythe processing circuitry, that indication will flag to the processingcircuitry that the pre-decoded instruction is potentially corrupted,allowing the processing circuitry to take appropriate action. Hence,incorrect operation resulting from execution of instructions that havebeen incompletely pre-decoded can be avoided without significantadditional cost and complexity in the design.

In the embodiments discussed above, an abnormal instruction identifieris added only to T32 instructions. For example, this can be done byadding sideband information to the first portion T32a of a T32instruction. An abnormal instruction identifier is not added to a T16instruction. To some extent, this is acceptable as the T16 instructionset is small enough that abnormal instructions can be decoded moreeasily than for T32 instructions, and so less power is consumed by thedecoding circuitry in decoding these abnormal instructions.

However, in one embodiment, the first portion T32a of a T32 instructionis extended when an incomplete pre-decode identifier is added. In orderto make best use of the memory space available, two T16 instructionsshould be the same length as one T32 instruction (or an ARMinstruction). Thus, as the first portion of the T32 instruction has beenextended to accommodate the incomplete pre-decode identifier, anadditional bit can also be added to the T16 instruction duringpre-decoding. This bit can be used to store an abnormal instructionidentifier. For example, the bit could contain sideband informationwhich indicates whether or not the instruction is an undefinedinstruction. In this way, the power consumed in decoding undefinedinstructions can be reduced.

The second portion T32b of a T32 instruction does not need an abnormalinstruction identifier because this information will be contained in thefirst portion T32a. However, it is important that the pre-decodingoperation performed on the T32b portion is the same as the operationperformed on a T16 instruction by the pre-decoding circuitry 10. Thus,if a block is speculatively identified as representing a T32b portion,then the pre-decoding circuitry 10 adds an abnormal instructionidentifier which indicates, provisionally assuming (against thespeculation) that the block represents a T16 instruction, whether or notthat T16 instruction is abnormal. Thus, the operation performed on ablock is the same regardless of whether it is identified as a T16instruction or a T32b portion. If the pre-decoding circuitry 10mistakenly identifies a T16 instruction as a T32b portion, then thisdoes not matter because the abnormal instruction identifier is added tothe block anyway, and so a pre-decoding error does not occur. If thespeculation was correct, then the abnormal instruction identifier in theT32b portion can be ignored.

FIG. 15 shows the format of a T16 instruction and a T32 instruction oncepre-decoding operations have been performed on them according to atleast some embodiments. For a block speculatively identified as a T16instruction, an instruction portion identifier is added in bit [16] andset to “0”, and a sideband bit is added in bit [17] indicating whetherthe instruction is abnormal. The T16 opcode in bits [15:0] isunmodified. The same operation is performed on a block speculativelyidentified as representing the second portion T32b of a T32 instruction.An instruction portion identifier is added in bit [34] and set to “0”,and a sideband bit is added in bit [35] indicating, while provisionallyidentifying the block as a T16 instruction, whether the T16 instructionis abnormal. The second half of the T32 opcode in bits [33:18] isunmodified.

For a block speculatively identified as the first portion T32a of a T32instruction, the pre-decoding circuitry 10 adds sideband information tobits [15:14] of the block. An instruction portion identifier is added inbit [16] and set to “1”, and an incomplete pre-decode identifier isadded in bit [17] indicating whether the T32 instruction spans a cacheline boundary.

Thus, the same pre-decoding operation is performed on a block regardlessof whether it is identified as a T16 instruction or as a T32b portion.The error-resistant pre-decode format used for instructions alsoconsistently utilizes extra bits which are available to convey anabnormal instruction identifier.

The process of step 202 in FIG. 11, in which blocks are pre-decoded bythe pre-decoding circuitry 10, is described in more detail withreference to FIG. 16. In step 250, the pre-decoding circuitry 10speculatively identifies which instruction portion a block represents.In step 254, the pre-decoding circuitry 10 detects whether or not theblock has been identified as the first portion T32a of a T32instruction.

If the pre-decoding circuitry 10 identifies the block as the firstportion T32a of a T32 instruction, then flow proceeds to step 260, inwhich the pre-decoding circuitry 10 detects whether the T32 instructionis an abnormal instruction. In step 262, the pre-decoding circuitry 10adds sideband information to the block depending on whether or not theT32 instruction is an abnormal instruction. In step 264, thepre-decoding circuitry 10 adds an instruction portion identifier in theform of an ID bit which is added at bit [16] of the block and set to“1”.

On the other hand, if at step 254 the pre-decoding circuitry 10 did notidentify the block as a T32a portion, then flow proceeds to step 270.The pre-decoding circuitry 10 provisionally assumes that the blockrepresents a T16 instruction, and detects whether or not that T16instruction is an abnormal instruction. Then, in step 272, independentof whether the block is a T16 instruction or the second portion T32b ofa T32 instruction, the pre-decoding circuitry 10 adds sidebandinformation to the block indicating whether or not the T32 is anabnormal instruction. At step 274, the pre-decoding circuitry 10 thenadds an instruction portion identifier in the form of an ID bit which isadded at bit [16] of the block and set to 0.

For both paths through the flow diagram in FIG. 16, flow then proceedsto step 280, in which any further pre-decoding operations are performedby the pre-decoding circuitry 10. This may include, for example, addingthe operation discussed in relation to FIG. 14 for adding an incompleteinstruction identifier. Once any further pre-decoding operations arecomplete, the pre-decoded blocks are then passed to the cache 20 in step284. Flow then returns to step 204 in FIG. 11.

Another problem that can occur within the data processing apparatus whenusing the pre-decoding circuitry is when there is a mix of instructionstates within a particular cache line. As a particular example, an ARMprocessor may be able to execute instructions from either the ARM,Thumb, ThumbEE or Jazelle instruction sets developed by ARM Limited,United Kingdom, and it is possible for one cache line to includeinstructions from more than one of the instruction sets.

At any particular point in time, the processing circuitry will be in aparticular processor state, dependent on which instruction set theinstructions currently being executed belong to. Hence, at one point intime, the processing circuitry may be in the ARM processor state, whilstat a subsequent point it may branch into a sequence of Thumbinstructions, and hence enter the Thumb processor state.

When pre-decoding instructions, the pre-decoding circuitry needs toassume a speculative processor state when performing the pre-decoding,and this chosen speculative processor state would then typically be usedfor pre-decoding a whole cache line's worth of instructions received viathe processor system interface 5. In one particular embodiment, thespeculative processor state is chosen to be the state of the processorat the time the pre-decoding operation is performed by the pre-decodingcircuitry. It will be appreciated that the instruction cache 20 couldstore pre-decoded instructions which have become corrupted if, forexample, the chosen speculative processor state is not an appropriateprocessor state for all of the instructions in the cache line. Forexample, if the processor is in Thumb state when the pre-decodingoperation is performed for a particular cache line, but the cache linemixes up ARM and Thumb instructions, then it will be appreciated thatthe ARM instructions may be pre-decoded incorrectly, resulting incorrupted pre-decoded instructions for those ARM instructions. Indeed,it is also possible for an entire cache line to be pre-decoded in thewrong state if the last instruction in one cache line is a statechanging branch instruction, and the next cache line is already beingsubjected to the pre-decoding operation by the pre-decoding circuitry.

Whilst it would be possible to mark every half word that is pre-decodedwith an indication of the speculative processor state used for thepre-decoding, this would consume a significant amount of area in theinstruction data RAMs of the instruction cache. Since it is rare thatcache lines mix state, in one embodiment the cache line is marked withthe state the processor was in when the cache line was pre-decoded, thisproving to be more area efficient. Accordingly, in one embodiment eachentry in the instruction tag RAMs of the cache can be extended to storethe speculative processor state as shown schematically in FIG. 18. Aswill be understood by those skilled in the art, each entry in aninstruction tag RAM will typically store an address portion 480, andwill also often store one or more other control bits, for example avalid bit to indicate whether the associated cache line is valid. Asshown in FIG. 18, an additional two bits can be provided for each entryto identify the speculative processor state 485 that was used whenperforming the pre-decoding operation on the corresponding cache line inthe instruction data RAM.

FIG. 17 illustrates in more detail components provided within theinstruction cache 20 when such an extended tag RAM entry is used withinthe instruction tag RAMs. As shown in FIG. 17, as the pre-decodedinstructions for a particular cache line are stored in that cache linewithin an instruction data RAM 450 of the instruction cache, thecorresponding entry in the relevant instruction tag RAM 460 is populatedwith the address portion 480 and the speculative processor stateinformation 485, which will have been output by the pre-decodingcircuitry 10.

When the processor subsequently issues an access request seeking to readan instruction from the instruction cache 20, the cache performs alookup operation using the address comparison circuitry 464, the statecomparison circuitry 468 and the hit/miss circuitry 470. In particular,the address comparison circuitry 464 determines whether the addressspecified by the access request matches any of the address portionsstored in selected entries within the instruction tag RAMs 460. In oneembodiment, the instruction cache 20 is arranged as an n-way setassociative cache, and in one particular embodiment takes the form ofthe two-way set associative cache shown in FIG. 19. Accordingly, in suchan embodiment there will be two tag RAMs 460-0 and 460-1, and anassociated two data RAMs 450-0 and 450-1. For each cache line 492, therewill be a corresponding entry 490 in the associated tag RAM for storingthe information shown schematically in FIG. 18.

It will be appreciated by those skilled in the art that when an accessrequest is issued by the processing circuitry, an index portion of theaddress is used to identify a particular set in the instruction cache,each set comprising one cache line in each way, and accordingly for theexample of FIG. 19 each set comprising two cache lines. A tag portion ofthe address specified in the access request is then compared with theaddress portion 480 stored in the corresponding entry in each tag RAM,and accordingly for the example of FIG. 19 the address comparison logic464 will perform two address comparisons, namely comparing the tagportion of the address with the address portion 480 from the entry ineach tag RAM that is associated with the set specified by the indexportion of the address.

Furthermore, in addition to performing the address comparison using theaddress comparison circuitry 464, state comparison circuitry 468 alsoperforms a comparison of the current processor state against thespeculative processor state 485 stored in those accessed entries in thetag RAM. The hit/miss circuitry 470 is arranged to only detect a hitcondition if both the address comparison circuitry 464 identifies amatch, and the state comparison circuitry 468 also identifies a match,i.e. a hit condition is only detected if the instruction seeking to beaccessed is present in the instruction cache and the speculativeprocessor state used to pre-decode that instruction matches the currentprocessor state.

In the event of a hit condition, the multiplexer 472 is caused to outputthe instruction read from the data RAM 450 of the appropriate way in theinstruction cache, from where that instruction is then provided to thedecoding circuitry 30.

If a hit condition is not detected, then a miss signal is issued overpath 474, causing a linefill request to be issued from the processorsystem interface 5. As a result, the relevant cache line of instructionswill be fetched from memory and passed through the pre-decodingcircuitry 10.

It will be appreciated that there are two types of miss condition. Thefirst is the classical one where the instruction seeking to be accessedis not currently in the instruction cache, and in this instance theoutput from the pre-decoding circuitry 10 can be allocated to theinstruction cache in the usual manner. For the two way set associativecache example discussed earlier, this will involve cleaning andinvalidating the cache line contents in one of the ways, and allocatingthe newly pre-decoded instructions to that cache line. However, anothermiss condition that can now be generated is where the address comparisoncircuitry 464 detects a match, i.e. the instruction is present in thecache, but the state comparison circuitry 468 does not detect a match,and hence identifies that the relevant instruction has been pre-decodedassuming a wrong processor state. In this instance, the cache misscauses a re-fetch of the cache line from memory, along with a re-pass ofthe fetched instructions through the pre-decoding circuitry. While inone embodiment the output from the pre-decoding circuitry 10 in such asituation could be allocated in exactly the same manner as discussedabove, in an alternative embodiment the output from the pre-decodingcircuitry 10 will preferentially be allocated to a different way. Thiscan avoid a potential “thrashing” problem that could otherwise arise insituations where the cache line contains instructions from more than oneinstruction set.

To illustrate this thrashing problem, consider the example where a firstpart of the cache line contains Thumb instructions and the second partof the cache line contains ARM instructions, with the whole cache lineinitially being pre-decoded assuming a Thumb processor state. It will beappreciated that if one of the ARM instructions is then accessed whilethe processing circuitry is in the ARM processor state, it will bedetected by the state comparison circuitry 468 that the speculativeprocessor state (i.e. the Thumb state) does not match the currentprocessor state (i.e. the ARM state), and accordingly the re-fetchprocess described above will be performed. This time, the entire cacheline will be pre-decoded assuming the ARM processor state, which nowprovides the correct pre-decoded ARM instruction that was the subject ofthe access request. However, if this output from the pre-decodingcircuitry was used to overwrite the previous cache line contents, thenif at a later stage the processing circuitry whilst in the Thumbprocessor state seeks to access one of the Thumb instructions in thefirst part of the cache line, it will be seen that a miss condition willagain be detected since the state comparison circuitry 468 will againidentify that the speculative processor state and the current processorstate do not match.

By allocating the output from the pre-decoding circuitry 10 to adifferent cache line in such situations, this problem can be avoided,since it allows both versions to co-exist in the cache. Accordingly,considering the earlier example, the same set of instructions forming aparticular cache line's worth of pre-decoded instructions will be storedin one cache line as a series of pre-decoded instructions that werepre-decoded assuming the Thumb processor state, and will be stored inanother cache line in a different way as a series of pre-decodedinstructions that were pre-decoded assuming the ARM processor state.

Whilst this avoids the above thrashing problem, when cleaning andinvalidating the cache, the clean and invalidate operation will need tooperate over multiple cache lines per physical address if theinstruction at the physical address has been stored in multiple cachelines due to the above-described procedure.

FIG. 20 illustrates the above-described pre-decoding operation of oneembodiment. At step 500, the pre-decoding operation starts, whereafterat step 504 the current processor state of the processing circuitry isdetermined and set as a speculative processor state. Thereafter, at step508, the pre-decoding operation is performed by the pre-decodingcircuitry 10 using the speculative processor state. Then eachpre-decoded instruction is stored in the cache 20 at step 510 along withan indication of the speculative processor state.

FIG. 21 illustrates schematically the process performed by theinstruction cache 20 upon receipt of an access request from theprocessing circuitry. At step 550, an access request from the processingcircuitry is awaited, and on receipt of such an access request a lookupprocedure is performed in the cache at step 554 using the addressspecified by the access request.

At step 558, it is determined whether the address comparison circuitry464 has detected an address match, and if not the hit/miss circuitry 470generates a miss condition at step 570, causing a linefill operation totake place.

If at step 558, an address match is detected, then at step 560 it isdetermined whether the state comparison circuitry 468 has detected amatch between the current processor state and the speculative processorstate stored in the relevant instruction tag RAM entry. If so, thehit/miss circuitry 470 generates a hit signal causing the matchingpre-decoded instruction to be output from the relevant data instructionRAM 450 via the multiplexer 472 to the decoding circuitry 30 of theprocessing circuitry 25. However, if at step 560 it is determined thatthe current processor state does not match the speculative processorstate, then a miss condition is generated at step 568 causing a linefilloperation to take place into a different cache way from the cache way inwhich the address match was detected by the address comparison circuitry464.

Often a state changing instruction used to trigger a change in processorstate will cause a branch to a new address (and hence a different cacheline) containing the first instruction to be executed in the newprocessor state. However, in an alternative embodiment, this may not bethe case, and hence following a state changing instruction the nextinstruction to execute may be the next instruction in the same cacheline. In such situations, a problem can arise when a fetch from theinstruction cache contains more than one instruction, and oneinstruction in that fetch (other than the last one) is a state changinginstruction. In this case a ‘hit’ will correctly occur for the fetch,but all instructions after the state changing instruction will have beenincorrectly pre-decoded in the case where the instruction cache holds acache line that has been pre-decoded in only one state. To address this,in one embodiment the error detection circuitry 35 is arranged to spotthis situation, to trigger a ‘state error’ for the sequentialinstructions after the state changing instruction, and to initiateeither a line fill or a loop back through the pre-decode process.

In a transition between the ARM and Thumb instruction sets thispotential problem will not occur since the state changing instruction isa BLX type instruction that branches to a new cache line. However in atransition between the Thumb and ThumbEE instruction sets, this problemcould arise since the state changing instruction (ENTERX/LEAVEX) doesnot use a branch and hence a fetch from a new address will not betriggered by the state changing instruction. In this case the errordetection mechanism described above can be used to identify the statechanging instruction, trigger a ‘state error’ for the sequentialinstructions after the state changing instruction, and then initiate anew fetch from the memory address immediately after the state changinginstruction.

Whilst in the above described embodiments the pre-decoder circuitry hasbeen associated with a level one instruction cache used to directlyprovide instructions to the processing circuitry 25, in an alternativeembodiment as shown in FIG. 22 the pre-decoder circuitry 10 may beassociated with a combined cache storing both instructions and data, forexample a level two cache 605. As shown in this figure, the processorsystem interface 5 actually communicates with the combined level twocache 605, rather than directly with memory, and for completeness thelevel one data cache 615 is also shown in addition to the instructioncache 20 discussed earlier. In this embodiment, instructions from memorywould be routed through the pre-decoder circuitry 10 to createpre-decoded instructions for storing in the combined level two cache605. However, data retrieved from memory to the level two cache, orwritten back to memory from the level two cache, will bypass thepre-decoder circuitry 10.

As shown in FIG. 23, an additional identifier 630 can be associated witheach cache line to identify whether the cache line stores instructionsor data. This will be added in addition to the speculative processorstate bits 635 and address portion 640 within each tag RAM entry 625,hence identifying for the corresponding cache line 650 whether thecontents of that cache line are pre-decoded instructions, or instead aredata values used by the processing circuitry when executing thoseinstructions. By such an approach, the instruction/data identifier 630enables those cache lines which have been subjected to pre-decoding tobe clearly identified. Accordingly, when performing lookup procedures inthe cache, it can be determined whether there is any need to checkspeculative processor state against current processor state during thelookup procedure, in particular such a check only being necessary forthose cache lines storing pre-decoded instructions.

From the above description, it will be seen that through storing of thespeculative processor state in the cache, it is possible to later detecta condition where the assumption of speculative processor state by thepre-decoding circuitry was wrong, and accordingly the pre-decodedinstruction read from the cache should not be used by the processingcircuitry. This provides a simple and effective, power efficient,mechanism for detecting certain situations where instructions have beencorrupted by the pre-decoding process.

FIG. 24 illustrates an instruction pipeline having a form similar tothose previously described. This instruction pipeline supportspre-decoding and subsequent execution of processing operationscorresponding to instructions of three different sets of instructions,namely ARM instructions, Thumb32 instructions, and Thumb16 instructions.It has been recognised that many common ARM instructions share or havefunctionality closely corresponding to instructions which are alsopresent within the Thumb32 instructions. This recognition may beexploited by using a system in which ARM instructions are pre-decoded bythe pre-decoding circuitry 10 to form pre-decoded instructions storedwithin the instruction cache 20 and using a shared format to representshared functionality with corresponding Thumb32 instructions. Thus, inthe de-coded instructions stored within the instruction cache 20, theindividual pre-decoded instructions corresponding to either ARMinstructions or Thumb32 instructions will share the same format torepresent the same functionality. The re-mapping performed on ARMinstructions by the pre-decoding circuitry 10 seeking to re-map theminto a form in which they are represented within the pre-decodedinstructions in the same way as Thumb32 instructions representsadditional activity at the pre-decoding stage, but this activity will beperformed when the instructions are being loaded into the instructionscache 20 whereas the pre-decoded instructions which have been re-mappedin this way will typically be read many times from the instruction cache20 and executed by the remainder of the system. Thus, the additionalactivity (energy) consumed in the re-mapping is amortised across manyuses of the re-mapped instructions.

As will be evident form FIG. 24, the re-mapping of the ARM instructionsinto a form similar to the Thumb32 instructions in their pre-decodedforms enables the Thumb32 decoding circuitry 45 to be re-used for themajority of the de-coding operations which need to be performed inrespect of what were originally ARM instructions. This saves gate countwithin the decoding circuitry and consequently saves energy.

There are some ARM instructions which do not share functionality closelyenough with any Thumb32 instruction that they can be re-mapped into ashared format and for which decoding circuitry is likely alreadyprovided within the Thumb32 decoding circuitry 45. In respect of theseARM-only instructions additional decoding circuitry 46 is provided andswitched into use when such ARM-only instructions are identified. ThisARM-only de-coding circuitry 46 is considerably smaller than a full ARMdecoding circuitry block which would be required in conventionalmultiple instruction set systems.

As previously described, the pre-decoded instructions are thirty sixbits in length. The original ARM instructions are 32 bit instructions.Thus, the additional bits within the pre-decoded instructions can beused to represent the previously discussed side band signals as is shownin FIG. 25. The provision of the same side band signals in respect ofARM instructions as well as the other instructions facilitates moreefficient decoding within the decoding circuitry 45, 50, 46. One bitwithin the pre-decoded instruction is used to flag ARM-only instructionsfor which there is no Thumb32 equivalent and for which the ARM-onlydecoding circuitry 46 is used to perform decoding and generateappropriate control signals therefrom. This ARM-only bit is selected asa bit having a fixed value within all Thumb32 instructions and which isset to the opposite value within pre-decoded instructions correspondingto ARM-only instructions.

One feature of the ARM instruction set is that every instructionincludes four bits of condition codes, namely codes N, C, Z, and V. Asshown within FIG. 25, these condition codes are allocated positionswithin the pre-decoded instruction corresponding to bit positions 16,17, 34, and 35. These are bit positions normally used for other purposeswithin pre-decoded instructions corresponding to Thumb32 instructions,namely identification bits, an incomplete bit, and a side band signal.Given that ARM instructions are stored as 32-bit aligned words inmemory, the identification and incomplete bits are not required for ARMinstructions within the pre-decoded instructions and only one area ofside band bits is necessary thereby freeing bitspace within thepre-decoded instructions for ARM instructions in order to represent thecondition codes N, C, Z, and V. The Thumb32 de-coding circuitry 45 canbe readily modified to respond to these condition codes withinpre-decoded instructions for ARM instructions in a manner which gatesthe execution of those pre-decoded instructions in dependence upon theassociated condition code values. If a pre-decoded instructioncorresponding to an ARM instruction meets its condition codes, then itwill be executed and appropriate control signals generated therefrom. Ifa pre-decoded instruction does not meet its condition codes then it willnot be executed.

FIG. 26 illustrates example of a Thumb32 ADC instruction and an ARM ADCinstruction both decoded into the form of a pre-decoded instruction inwhich shared functionality is represented in a shared format.Corresponding fields within the Thumb32 instruction and the ARMinstruction have been given the same names. It will be seen that whilethe formats of the original Thumb32 instruction and the original ARMinstruction are very different, they nevertheless contain nearly all ofthe same fields and accordingly re-mapping the fields within the ARMinstruction into positions within the pre-decoded instructioncorresponding to the same position of those fields when a Thumb32instruction is pre-decoded is relatively simple and efficient. Thisre-mapping is performed by the pre-decoding circuitry 10 illustrated inFIG. 24.

The ARM instructions include four condition code bits at bit positions[31:28]. As previously discussed, these condition codes are mapped tobit positions 35, 34, 17, and 16 within the pre-decoded instructioncorresponding to an ARM instruction. The pre-decoded instructioncorresponding to the ARM instructions still contains side band signalsat bit positions [15:14], but these will apply to the whole of the 36bits of pre-decoded instruction since ARM instructions are 32-bitaligned in memory and accordingly the issues associated withinstructions spanning cache line boundaries do not arise in relation toARM instructions since the cache lines are word aligned. Also of notewithin FIG. 26 is bit position 33 within the pre-decoded instructions.This is shown as having a value of “should be 0” in respect of both thepre-decoded instructions corresponding to the Thumb32 instruction andthe ARM instruction. This “should be 0” value corresponds to bit 31within the Thumb32 instruction. If bit 31 within the Thumb32 instructionis not a zero, then this is defined as having unpredictable behaviour.

FIG. 27 illustrates how the unpredictable behaviour associated with the“should be 0” value within the pre-decoded instruction being set to a“1” can be used to provide instruction bitspace within the pre-decodedinstruction to represent functionality of the ARM instructions which isnot shared by any Thumb32 instructions. In the example of FIG. 27, theARM instruction for which there is no Thumb32 equivalent is a ADCinstruction in it register-shifted register variant. In this variant ashift register Rs specified in bits [11:8] of the ARM instruction storesa value representing a shift amount to be applied to one of the valuesstored within the two input source registers Rm, Rn. Thumb32instructions do not have register-shifted register variants of their ADCinstruction. The remainder of the fields within the ARM instruction ofFIG. 27 are similar to those illustrated in FIG. 26. However, theregister-shifted register variant of the ARM instruction does notinclude an immediate field. This immediate field was mapped to bits[32:30] and [23:22] within the pre-decoded instructions for both theThumb32 and ARM instructions of FIG. 26. This bitspace within thepre-decoded instructions can be re-used to represent the registeridentifier Rs of the register storing the shift value within theregister-shifted register version of the ARM instruction for thepre-decoded instruction form of FIG. 27. The decoding circuitry 45 isconfigured to recognise the pre-decoded instruction supplied to it ashaving this particular form by the setting of the “should be 0” bit 33to a value of “1” in the pre-decoded instruction representing an ADC ofa register-shifted register variant ARM instruction.

This behaviour whereby bitspace corresponding to unpredictable behaviourfor the Thumb32 instructions is re-used to represent ARM-only variantinstructions can be contrasted with the use of the ARM-only bit at bitposition 13. The ARM-only bit at bit position 13 can be reserved forinstructions with much less similarity to any Thumb32 instructions andfor which the ARM-only decoding circuitry 46 is provided. Re-using thebitspace corresponding to the unpredictable behaviour in Thumb32instructions not only preserves instruction bitspace within thepre-decoded instructions it also facilitates re-use of the same gateswithin the shared portion of the decoding circuitry 45.

FIG. 28 illustrates a further feature which may be provided by thepre-decoding circuitry 10. It has been recognised that sequences ofThumb16 instructions can include adjacent instructions specifyingcombined functionality corresponding to a single Thumb32 instruction.The pre-decoding circuitry can recognise this by pre-decoding theadjacent Thumb16 instructions into a single pre-decoded instructioncorresponding to the equivalent Thumb32 instruction. The singlepre-decoded instruction corresponding to the Thumb32 instruction will beissued as a single entity into the execution pipeline and accordinglyexecuted more rapidly and efficiently then would two separate Thumb16instructions. The pre-decoding circuitry 10 can concentrate on arelatively small number of common cases where two adjacent Thumb16instructions may be re-mapped to a corresponding pre-decoded form of aThumb32 instruction in order to balance the benefit and cost ofproviding the additional circuitry to identify such opportunities.

FIG. 29 is a flow diagram schematically illustrating the re-mapping ofARM instructions to Thumb32 instructions within the pre-decodingcircuitry. At step 700, a cache line is fetched from memory. At step702, a determination is made as to whether or not the first instructionto be de-coded is from the ARM instruction set. If the instruction to bepre-decoded is not from the ARM instruction set, then processingproceeds to step 704 at which pre-decoding in accordance with either aThumb16 or Thumb32 instruction is performed.

If the determination at step 702 was that the instruction to be decodedis an ARM instruction, then step 706 determines whether the ARMinstruction can be mapped to a T32 format pre-decoded instruction fordecoding with a shared portion of the decoder. If the ARM instructioncan be mapped in this way then processing proceeds via step 707. If theARM instruction cannot be mapped in this way the step 709 pre-decodesthe ARM instruction into a pre-decoded instruction with the ARM-only bitset so that a non-shared portion of the decoder will be used. Step 708then stores the pre-decoded instructions in the cache memory 20. Step710 determines whether or not there are more instructions in the fetchedcache line fill which pre-decoding is required. If there are more suchinstructions, then processing returns to step 702, otherwise theprocessing halts.

FIG. 30 is a flow diagram schematically illustrating how two Thumb16instructions may be re-mapped into a pre-decoded instructioncorresponding to a single Thumb32 instruction. At step 712 a cache lineis fetched from memory. At step 714 a determination is made as towhether or not the first instruction to be decoded is a Thumb16instruction. If the instruction is not a Thumb16 instruction, thenprocessing proceeds to step 716 where the pre-decoding is performed inaccordance with the appropriate behaviour for an ARM or a Thumb32instruction.

If the determination at step 714 was that the instruction fetched is aThumb16 instruction, then step 718 reads the next Thumb16 instructionand step 720 identifies whether the two adjacent Thumb16 instructionscorrespond to one Thumb32 instruction.

If the determination at step 720 was that the two adjacent Thumb16instructions do not correspond to one Thumb32 instruction, then step 722generates pre-decoded instructions for two separate Thumb16instructions. However, if the determination at step 720 was that the twoadjacent Thumb16 instructions do correspond to one Thumb32 instruction,then step 724 generates a pre-decoded instruction corresponding to theequivalent Thumb32 instruction as opposed to the two separate Thumb16instructions that would be generated by step 722.

Step 726 stores the pre-decoded instructions into the instruction cache20. Step 728 then determines whether or not there are more instructionswithin the fetched cache line that require pre-decoding. If there aremore such instructions then processing returns to step 714.

FIG. 31 illustrates a flow diagram corresponding to processing which canbe performed within the pre-decoding circuitry 10 to identify branchinstructions and early terminate pre-decoding operations. If a branchinstruction which either will be taken, or is predicated taken, isencountered, then it can be energy efficient to early-terminatepre-decoding as the energy expended upon such pre-decoding is wastedsince it is likely that instructions subsequent to the branch (orpredicted taken branch) will not be executed. At step 730, a cache lineis fetched from memory. At step 732, the pre-decoding circuitry 10identifies whether or not the instruction being pre-decoded is a takenbranch. If the instruction being pre-decoded is a taken branch (e.g. anunconditional branch (jump)), then processing terminates. If thedetermination at step 732 is that the instruction is not a taken branch,then the instruction is pre-decoded at step 734. Step 736 then storesthe pre-decoded instruction within the instruction cache 20 and step 738determines whether or not there are more instructions within the cacheline which required pre-decoding. If there are more such instructionsthen processing returns to step 732.

It will be appreciated that the processes illustrated by the flowdiagrams of FIGS. 29, 30, and 31 are illustrated as sequential processesand processes which are independent of one another. Those familiar withthis technical field will realise that when these processes areimplemented using the pre-decoding circuitry 10, these processes maywell occur with a different ordering of steps or with some steps beingperformed in parallel. Furthermore, the processes which are illustratedseparately in FIGS. 29, 30, and 31 will likely in practise be performedat least partially in parallel. The present techniques encompass all ofthese various alternatives.

Although particular embodiments have been described herein, it will beapparent that the invention is not limited thereto, and that manymodifications and additions may be made within the scope of theinvention. For example, various combinations of the features of thefollowing dependent claims could be made with the features of theindependent claims without departing from the scope of the presentinvention.

1. A data processing apparatus comprising: processing circuitry for executing a sequence of instructions; pre-decoding circuitry for receiving the instructions fetched from memory and performing a pre-decoding operation to generate corresponding pre-decoded instructions; a cache for storing the pre-decoded instructions for access by said processing circuitry; wherein the pre-decode operation, for each instruction fetched from the memory, detects whether the instruction is an abnormal instruction and upon such detection provides in association with a corresponding pre-decoded instruction an identifier identifying that instruction as abnormal.
 2. A data processing apparatus as claimed in claim 1, wherein said abnormal instruction is an undefined instruction.
 3. A data processing apparatus as claimed in claim 1, wherein said abnormal instruction is an unpredictable instruction.
 4. A data processing apparatus as claimed in claim 1, wherein said identifier comprises side band bits in said corresponding pre-decoded instruction.
 5. A data processing apparatus as claimed in claim 4, wherein said side band bits are appended to said corresponding pre-decoded instruction.
 6. A data processing apparatus as claimed in claim 4, wherein said side band bits are inserted into said corresponding pre-decoded instruction.
 7. A data processing apparatus as claimed in claim 4, wherein said side band bits replace redundant bits in said corresponding pre-decoded instruction.
 8. A data processing apparatus as claimed in claim 2, wherein said identifier identifies said abnormal instruction as undefined.
 9. A data processing apparatus as claimed in claim 3, wherein said identifier identifies said abnormal instruction as unpredictable.
 10. A data processing apparatus as claimed in claim 1, wherein said identifier identifies said abnormal instruction as undefined irrespective of a type of the abnormal instruction.
 11. A data processing apparatus as claimed in claim 1, wherein the pre-decode operation converts said abnormal instruction into a predetermined undefined instruction.
 12. A data processing apparatus as claimed in claim 1, wherein upon encountering said corresponding pre-decoded instruction said processing circuitry is configured to execute said abnormal instruction in a predetermined manner.
 13. A data processing apparatus as claimed in claim 12, wherein said predetermined manner comprises executing said corresponding pre-decoded instruction such that a state of the data processing apparatus is not altered.
 14. A data processing apparatus as claimed in claim 12, wherein said predetermined manner comprises triggering an exception.
 15. A method of processing data comprising the steps of: fetching instructions from memory; performing a pre-decoding operation on said fetched instructions to generate corresponding pre-decoded instructions, wherein said pre-decoding operation comprises, for each instruction fetched from the memory, detecting whether the instruction is an abnormal instruction and upon such detection providing in association with a corresponding pre-decoded instruction an identifier identifying that instruction as abnormal; and storing the pre-decoded instructions in a cache for access by said processing circuitry.
 16. A data processing apparatus comprising: processing means for executing a sequence of instructions; pre-decoding means for receiving the instructions fetched from memory means and performing a pre-decoding operation to generate corresponding pre-decoded instructions; cache means for storing the pre-decoded instructions for access by said processing means; wherein the pre-decode operation, for each instruction fetched from the memory means, detects whether the instruction is an abnormal instruction and upon such detection provides in association with a corresponding pre-decoded instruction an identifier identifying that instruction as abnormal. 